Fix typos in slice-expression JEPs (#146)

Author: gibson042

jep-005-array-slices.md

@@ -32,7 +32,7 @@ etc)
 3. Extracting only elements before a given stop position: `[:5]`
 
 
-4. Extracting elements between a given start and end position: `[2::5]`
+4. Extracting elements between a given start and end position: `[2:5]`
 
 
 5. Only the last 5 elements: `[-5:]`

jep-015-string-slices.md

@@ -10,7 +10,7 @@
 
 ## Abstract
 
-The original [JEP 5](https://github.com/jmespath-community/jmespath.spec/blob/main/jep-005-array-slices.md) introduced `slice-expression` in the grammar to slice specific portions of an array. While the syntax was specifically designed to operate on strings, there is no actual limitation in the grammar to extend this behaviour to string.
+The original [JEP 5](https://github.com/jmespath-community/jmespath.spec/blob/main/jep-005-array-slices.md) introduced `slice-expression` in the grammar to slice specific portions of an array. While the syntax was specifically designed to operate on arrays, the syntactic grammar allows it after any expression.
 
 This JEP introduces changes to allow `slice-expression` to operate on string types and act like a more powerful `substring()` function.